There are many competing view concerning what mechanical factors control how the heart fills with and ejects blood. Different investigators have focused on elastic or viscoelastic forces in the myocardium, relaxation from the previous systole, or chamber shape changes as being important determinants of how the heart fills with blood. Others have focused on interaction between the two sides of the heart, the pericardium, or the lungs as important factors. I am working to develop an integrated description of cardiac function that accounts for all these factors and can be used to assess the precise role and relative importance of each one in determining how the heart relaxes, fills, and ejects.